Combustion analyzers are used to determine the concentration of one or more components of a sample, by combusting the sample and analysing the gaseous products for specific oxides. Typically, the carbon, sulphur and/or nitrogen content of the sample is measured by detecting CO2, SO2 and NO, respectively.
A schematic illustration of a typical combustion analyzer is shown in FIG. 1. The combustion analyzer 10 comprises a sample introduction stage 20, a combustion stage 30, a conditioning stage 40, and a detection stage 50. The sample introduction stage 20 comprises a sample introduction apparatus 22, to which are connected a supply of a sample 24, a supply of oxygen 26 and a supply of argon 27. The sample introduction apparatus 22 introduces these fluids into a combustion tube 32 in a suitable form for combustion to take place. A further supply of oxygen 25 may be provided, directly into the combustion tube 32. The combustion tube 32 is heated by an electric heater 34, so that the sample is delivered into an oxygen-rich atmosphere at high temperature, typically around 1000° C. The sample is thereby converted into various combustion products, such as CO2, H2O, SO2, NOx, etc. The combustion products leave the combustion tube 32 and pass through the conditioning stage 40, where processes such as cooling, filtering, drying, etc. take place. The conditioned products then pass through one or more dedicated detectors 52, 54, in which properties of the components of the combustion products may be detected. For example, CO2 may be detected by absorption of infrared radiation, using a non-dispersive infrared (NDIR) detector; SO2 may be detected by fluorescence with ultraviolet light, using a light sensor; and NO can be detected from de-excitation processes following its reaction with ozone (O3) to form excited NO2, using a chemiluminescence light sensor. The detected signals are indicative of the respective amount of each component of the combustion products and can therefore be related to the composition of the original sample. Finally, the detected combustion products are passed out of the detection stage 50, as waste products 56.
The performance of such a combustion analyzer 10—in terms of its suitability, reliability, accuracy and robustness—depends strongly on the performance of the combustion tube, in particular its ability to convert the element(s) of interest in a sample into its/their respective oxide(s).
Various combustion tube forms are known. One type of prior art combustion tube is simply a single, straight tube, such as the combustion tube 60 of the hydrogen sulphide analyzer 58, shown in FIG. 2. A supply of a sulphur-containing sample 62 and a supply of air 64 are provided to the combustion tube 60 at one end. The sample and air pass down the combustion tube 60, which extends within a heater 66, and the sample is combusted in the combustion tube 60 to produce sulphur dioxide. The downstream end of the combustion tube 60 opens into a hydrogenation chamber 68, which also extends within the heater 66. The sulphur dioxide passes into the hydrogenation chamber 68, along with a supply of hydrogen 70, and the sulphur dioxide is reduced to hydrogen sulphide (H2S). The hydrogen sulphide leaves the hydrogenation chamber 68 at a downstream end and passes out of an outlet pipe 72 to a lead acetate tape detector (not shown).
The combustion tube 60, while very simple, does not have good gas-mixing properties; that is, the sample and air flowing through the combustion tube 60 are not encouraged to mix, so localised regions of incomplete combustion may result. Given the limited opportunity for a sample to be combusted in the combustion tube 60 before entering the hydrogenation chamber 68 (or any other downstream processing device, if the combustion tube 60 is used with a different analyzer) and the poor mixing performance of the tube, a single, straight-sided tube has a number of disadvantages.
To mitigate the disadvantages of such a combustion tube, it is known to provide a catalyst inside the combustion tube, to promote sample combustion. It is also known to place a number of quartz beads inside the combustion tube, to promote mixing of the sample and oxygen. However, catalysts degrade over time and tend to promote oxidation of some components better than others, so catalysts need to be replaced regularly and analyzers need to be re-calibrated often. Also, while the beads work to some extent, any movement of them, resulting from movement of the combustion tube, will change the characteristics of the tube and therefore its combustion efficiency, again requiring re-calibration of the analyzer.
One alternative approach is proposed in U.S. Pat. No. 3,909,202. Here, a combustion tube comprises two tubes arranged one inside the other and surrounded by an outer shell. A sample for combustion is injected into the inner tube and passes through the diverting tube and into the outer shell. The gaseous combustion products are then delivered to an analyzer through an outlet opening in the outer shell.
Another approach, which is currently in use, is shown in FIG. 3. This combustion tube 74 is known as a “turbo tube” and is used in nitrogen, sulphur and halogens (NSX) combustion analyzers. The turbo tube 74, which is made of quartz, has an inlet port 76 for receiving supplies of a sample and oxygen 78,79. The inlet port leads into a primary chamber 80, which itself leads into a so-called turbo chamber 82. The two chambers are separated by a partition 84, which comprises an opening 86. The opening leads into a helical tube 88, which extends from the partition on its downstream side, i.e. into the turbo chamber 82. At the downstream end of the turbo chamber 82, there is an inlet tube 90 through which a further supply of oxygen 92 is provided, into the turbo chamber. There is also an outlet tube 93, which extends from outside the turbo chamber 82, into the chamber and through the space defined within the helical tube 88, towards the partition 84.
In use, a sample 78 is introduced into the primary chamber 80 along with a flow of oxygen 79. The turbo tube 74 is heated to a high temperature, which effects thermal cracking and combustion of the sample in the primary chamber 80. The combustion products pass into the turbo chamber 82 through the helical tube 88, which opens into the chamber towards its downstream end. The further supply of oxygen 92 is added to the combustion products with the aim of achieving complete combustion of the as-yet-uncombusted sample components. The final combustion products 94 flow out of the turbo tube 74 along the outlet tube 93, to gas conditioning and detection stages (not shown).
Although the above two combustion tubes may perform satisfactorily in practice, a number of disadvantages can nevertheless be identified. Firstly, both combustion tubes have a relatively high space demand; for example, to accommodate the various inlet and outlets ports and tubes at each end of the combustion tubes. The turbo tube, in particular, is relatively long, at around 40 cm, and needs a relatively bulky heater to surround it. These factors lead to a relatively large footprint for a combustion analyzer using one of these combustion tubes.
Secondly, the gas mixing capabilities and flow characteristics of the combustion tubes are not especially favourable. The helical tube 88 of the turbo tube 74 does effect some gas mixing, due to the difference in the speed of gas flowing on the inside and on the outside of the helix. However, at the gas flow rates used in combustion analysis, the fluid flow in the combustion tubes tends to be generally laminar and little mixing occurs. Poor sample and oxygen mixing can result in local oxygen deficiencies, leading in some circumstances to the production of soot. If combustion products including soot are allowed to leave the combustion tube and pass downstream to the gas conditioner(s) and/or detector(s), these devices will become contaminated and in need of manual cleaning.
Thirdly, it takes a relatively long time to purge, or flush out, the combustion products from the combustion tubes, following combustion of a sample, before another sample may be combustion analyzed. For both combustion tubes, the time required to flush them out may be as much as around 2 minutes. With the turbo tube 74, this disadvantage is exacerbated by the differences in density between the combustion products and the oxygen carrier gas. The more dense combustion gases—mainly CO2 and H2O—tend to flow along the bottom of the combustion tube, while the less dense oxygen flows along the top. This phenomenon can result in erratic gas flow behaviour in the combustion tube 74, especially at the beginning or end of a combustion procedure. This problem also applies to the U.S. Pat. No. 3,909,202 combustion tube, in particular as a result of the provision of an additional combustion chamber 8 at the bottom of the outer shell, for pre-combustion of the air carrier gas.
Accordingly, it would be desirable to provide an improved or alternative—in particular, a more combustion efficient—combustion tube and method for combustion analysing a sample. This invention aims to provide a combustion tube and method for combustion analysing a sample which address some or all of the above problems.